The present invention relates to a process for digesting garbage or garbage contained wastes, particularly to a microbiological treatment which facilitates the treatment with high efficiency and economics.
Although presently garbage and garbage contained wastes are being incinerated or reused for land reclamation, they are causing various secondary pollution troubles, as it is well known.
In this regard, such organic wastes as excess activated sludge and human wastes have so far been treated by an anaerobic digestive method. Recently, however, there is an indication that even garbage contained wastes are to be treated by an anaerobic digestive method. This anaerobic digestive method possesses such advantages as that it enables a reuse of the by-product methane gas as energy for operating the digestion facilities, and that it enables an effective use of digested sludge as a useful organic fertilizer. On the other hand, as a mechanism of anaerobic digestion, mainly two reactions are known. Namely, one is a liquefaction reaction wherein such volatile fatty acids as acetic acid, propionic acid and n-butyric acid are obtained by turning the organics involved in waste water into low molecular weight substance through the action of anaerobic liquefaction bacteria (septic bacteria), and the other is a reaction wherein these fatty acids thus produced are converted into methane by the action of gasification bacteria (methane bacteria). Thereagain, the usually exercised anaerobic digestion follows a process wherein the treatment is performed under a coexistent state of these two bacteria groups in the same tank for an extended period of time as long as 30 to 50 days. On account of this, in spite of such aforementioned non-polluting and energy-saving characteristic features, its actual application has been declining in number from year to year, to the extent where it is presently employed only for treating human wastes a few other purposes.
Quite recently, its aforementioned advantageous points are being re-evaluated, and studies are progressing in the U.S.A. and some other countries, in order to improve its low treatment efficiency which is the primary drawback of the process. Lately, it has been evidenced that the above two reactions can be separated from each other, and it is possible to shorten the treatment period significantly from that of the conventional parallel-dual fermentation by optimizing each of these two reactions. Incidentally, however, under the aforementioned two-step treatment process, the liquefaction reaction at the first step proceeds within a state of a weak acidic to neutral pH, but, because of the conversion of organic substances into volatile fatty acids in the course of the treatment process, it necessitates an amount of alkali, as a neutralizer, almost equivalent to the volume of organic acid generated. Any use of neutralizer in the course of the liquefaction process results in a lower separability of digested sludge and separated water at the final step. Moreover, if garbage is treated at the liquefaction process of the first step, sometimes it generates hydrogen gas in addition to carbon dioxide, and the volume of the generated hydrogen gas, in some cases, reaches 0.1 m.sup.3 per each kg of the charged organics. If the hydrogen gas is generated at the liquefaction process of the first step, much chemical energy would be lost at the liquefaction process, and it naturally affects the methane yield at the gasification of the second step.